1. Field of the Invention
The present invention relates to an image processing apparatus for subjecting color image data including a character to an image processing, and a computer-readable recording medium which records a program for performing the image processing.
2. Description of the Related Art
In recent years, it has become so popular to use a scanner apparatus designed to read a color image from a document so as to create an electronic file containing data about the color image, and transmit the electronic file to an external terminal unit, such as a personal computer, which is connected to the scanner apparatus via a network or a communication interface, and a complex machine having such a scanner function and a copy function.
With a view to reduction in communication load and in storage space required for storing image data in the form of an electronic file, the scanner apparatus or the complex machine is designed to compress color image data read from a document so as to reduce a file size. As a methodology for efficiently compressing image data, there has been known an irreversible compression (i.e., glossy compression) scheme suitable for compression of a multivalued image, such as a JPEG (Joint Photographic Experts Group) scheme, which is configured to irreversibly compress image data while keeping deterioration in image quality at an invisible level by taking advantage of human's visual characteristics, so as to perform high-efficiency data compression.
In an operation of compressing image data consisting of a mixture of a character and a background image, using the irreversible compression scheme, there is a problem that severe deterioration in image quality occurs in an area having a high contract, such as a character area, to cause a decline in visibility of the character. In order to improve the problem about poor visibility of a character mixed with a multivalued image, the following technique has been known.
In this conventional technique, only a character area is extracted from a color image read from a document, and formed as monochrome (i.e., achromatic) binarized image data. The obtained binarized image data is compressed using an reversible compression scheme suitable for compression of binarized data, such as an MMR (Modified Relative Element Address Designate) scheme. In addition, information about a specific color exhibiting a peak in a color histogram of the character area is acquired as color information of the character area. Image data about the remaining area other than the character area is compressed using the high-efficiency irreversible compression scheme.
That is, the above conventional technique is designed to compress image data about a character area and image data about a non-character area individually, and additionally acquire color information of the character area, so as to reduce a data volume without a decline in visibility of the character area (see, for example, JP 2002-77633A). There has also been known a file format, such as a high-compression PDF (Portable Document Format) or an XPS (XML Paper Specification) format, which is capable of combining the image data/color information of the character area and the image data about the remaining area other than the character area, and creating a single electronic file containing the combined data.
However, in cases where a specific color exhibiting a peak in a color histogram of a character area is used as a representative color of the character area as mentioned above, color reproducibility of a color character will deteriorate. This causes a problem that a delicate difference in hue cannot be rendered, for example, in a color image where a vermilion seal is stamped beside a red character.